Natural bathroom with rocket mass heater and foyer

Hey friends!

I need some help on this one. Bear with me, it's long, and I have nary a clue what I am doing.

I am looking to build a small building cheaply and with natural materials. We live in an RV, permanently parked on the edge of our land. We've been having heating issues, because the woodstove is just too damn small, so it doesn't burn for more than 2 hours (and that's if we are lucky), but anything bigger wouldn't work for the space, and would probably overheat us. So I want a rocket mass heater.

I also want a separate bathroom. The RV washroom, as with most RVs, is directly between the bedroom and the kitchen. It stinks!! And emptying the tanks sucks!! And even with 4-gallon maximum showers, the grey water tank fills too fast

All that to say, I want this building to address both of these issues. I want it to connect directly to the doorway of the RV, with a small foyer/mudroom kinda deal, and then have a medium-sized bathroom with a composting toilet (simple bucket system with a hatch on the outside to remove buckets from under the seat), and a shower. The rocket mass heater would basically live between the two spaces, leaving a bench for taking off boots on the foyer side, and a nice heated bench for the shower area on the other side. The wall between would be built around and on top of the bench portion, if that makes sense.
I have a basic idea of shape and layout, but where I'm lost is materials and construction. My first though was cob, and then I had a brief hope I could make it a wofati kinda deal, but that won't work in the spot, and we're looking more for the RMH than the thermal inertia, so I'm back to cob.

It is VERY humid here. I've seen cob done in humid areas, but with the added humidity of an open shower... Is this going to be too much? Has anybody done a cob bathroom in a coastal climate before? Can I do cob for the floor of a shower?
I love the look of cob, honestly. It would make for a peaceful bathroom imo.
My other question with cob would be what to use for the insulation, because I don't want to do straw, as I don't want the walls to be too thick, but I need lots of insulating.

Why I loved the wofati concept was that all the materials could come from the land. We have a lot of dried and drying pine logs of varying sizes. I want to use the bigger ones for a log cabin next year, but if I was able to use the smaller diameter logs for this, I'd be very happy. Are there any detriments to using these as the walls underneath cob? Like, stacking them log-cabin style, and doing cob over them? Would this insulate well and eliminate need for more insulation?

Is this all a terrible idea anyway? RMH in a bathroom?
Do you have any suggestions or guidance? I've not built very much in my life, so I'm basically rawdogging this whole project. Any guidance is helpful

There are a few points that you list as important;
- small size
- cheap
- need heat
- mudroom
- want  cob but not thick
- insulated
- peaceful
- deal with humidity well.
Listen I think world peace may be easier to achieve!!
BAD NEWS
Cob is not good as a shower base unless you want a mud pie making pit?
Cob has to be think to actually work, at least 12 inches.
GOOD NEWS
- Play Fleetwood Mac in the bathroom
- What size bathroom and mud room?
- Humidity is best dealt with movement of air, so a tall wind tower may pull humidity out , along with any heat perhaps.
- Can the heater be in the mudroom?
- Posts could be set in the ground at least 2 feet deep, what tools and equipment do you have?
- Log beams can be put on the top of poles.
- log cabin style would work, but post and beam with cob infill may work for you.
- Shower base should be either concrete or an old base.
- Shower walls could be iron sheets, corrugated or old glass panels.

East Cnada is big, where are you and whats the weather like?

John C Daley wrote:There are a few points that you list as important;
- small size
- cheap
- need heat
- mudroom
- want  cob but not thick
- insulated
- peaceful
- deal with humidity well.
Listen I think world peace may be easier to achieve!!
BAD NEWS
Cob is not good as a shower base unless you want a mud pie making pit?
Cob has to be think to actually work, at least 12 inches.
GOOD NEWS
- Play Fleetwood Mac in the bathroom
- What size bathroom and mud room?
- Humidity is best dealt with movement of air, so a tall wind tower may pull humidity out , along with any heat perhaps.
- Can the heater be in the mudroom?
- Posts could be set in the ground at least 2 feet deep, what tools and equipment do you have?
- Log beams can be put on the top of poles.
- log cabin style would work, but post and beam with cob infill may work for you.
- Shower base should be either concrete or an old base.
- Shower walls could be iron sheets, corrugated or old glass panels.

East Cnada is big, where are you and whats the weather like?

Thanks for your reply, John! Now where to start...

The heater would be mostly in the mudroom, but with part of the bench in the bathroom, if that makes sense and is possible. A cob wall would split the bench down the middle.

The measurements would be about 7'-8' wide, with the mudroom about 5' deep, bathroom another 10'. So whole building would be about 7'x15'.

In terms of tools and equipment, we've got drills/electric screwdriver, various saws (circular, jig, reciprocating, chain, handsaws, compass saw), sawhorses, carving tools, hammers, mallet, nails, screws, and all the other little things like caulking guns, things for spreading plaster, steal brushes etc.
We actually have a lot to work with, but no big excavators or anything like that.

When you say cob doesn't work unless it's 12"... I've seen cob done on strawbales, and earthbags mostly. 12" on either side of the wall?! Or 12" total including the innards? Hm...

We're in NB lowlands. Generally speaking: lots of rain and clouds. Temp range between 35°C and -30°C, though most of summer is around mid-twenties, and winter I'm not sure. Currently having a very very mild november-december, so god knows lol.
Humidity readings I've been getting inside the RV are usually 60-90% (currently 71%, low for the last 24hrs was 61%, high was 87%)

A wind tower sounds a bit more complicated than I intended, but it's an interesting idea. How tall are we talking here?

OK, when you see cob on strawbales, its not cob.
It is a clay plaster, and very different processes are used. Cob or adobe or mudbrick [ Australia ] all use clay in a block form.
Adobe and mudbrick create a dry solid brick to use and cob uses a wet lump to use.

I note you did not comment on the choice of music offered?
Buildings alway start at the base.
- foundation
- poles
- roof beams
- roof lining
- walls and windows
- RMH
- finishing
FOUNDATIONS Assuming a level site, either strip footings of rocks or concrete or a complete concrete slab with edge and thickening beams.
- Posts are set in the ground initially and the foundation built around them, on a 8 foot grid.
- concrete is a lot easier to work withe but you need trenches for shower and bath drains in place before you poor.
- install roof beams on poles with overhang as required. Remember the 7 x 15 is the internal wall dimensions.
- cut roof and ceiling battens to suit, insulate and use a metal roofing system.
Think about catching rainfall for domestic use.
- if there are door openings to bathroom or RV, an extra post in the doorway gives you a solid door frame.
- The foundation also needs to be under the dividing wall of the room.
- the RMH and the wall can be built together.